I am trying to simulate a mixed convection flow in a duct. Please see the attached document for the geometry and small explanation of the problem. In CFX, I can't apply Neumann type BC at the flow outlet. If I choose the boundary as outlet, I don't get option to apply BC on temperature. On the other hand, if I choose the boundary as opening, I get only two options 1) Opening temperature and 2) Static temperature. Could someone please suggest me how can I apply Neumann boundary condition at the outlet.

You cannot apply Neumann conditions on an outlet. The implementation in CFX is better in that it lets the fluid convect out of the domain so nothing needs specifying other than pressure - this is a superior approach anyway.